1. Field of the Invention
The present invention relates to an angular velocity sensor used for posture control, navigation, or the like of a mobile unit such as an airplane, an automobile, a robot, a ship, and a vehicle.
2. Description of the Related Art
Hereinafter, an angular velocity sensor in the related art will be described with reference to the drawings. FIG. 10 is a plan view of a transducer in the angular velocity sensor in the related art. FIG. 11 is a block diagram of the angular velocity sensor. FIG. 12 shows wave form charts of input and output signals regarding respective electrodes while the angular velocity sensor is in an operating state.
Referring to FIG. 10 and FIG. 11, the angular velocity sensor includes a tuning fork type transducer 3 having a pair of arm portions 2 at a shaft portion 1 and formed of a piezoelectric element and a control circuit portion 4 that drives the transducer 3 and detects the angular velocity applied to the transducer 3.
The transducer 3 includes a drive electrode portion 6 into which is inputted a drive signal 5 to oscillate the transducer 3 at a specific frequency, a monitor electrode portion 8 that detects the oscillation frequency of the transducer 3 and outputs the detected oscillation frequency as a monitor signal 7, and a sense electrode portion 10 that detects the angular velocity applied to the transducer 3 and outputs the detected angular velocity as sense signals 9 synchronized with the monitor signal 7. The drive electrode portion 6 and the sense electrode portion 10 are formed at the arm portions 2 and the monitor electrode portion 8 is formed in proximity to the boundary between the shaft portion 1 and the arm portions 2.
The control circuit portion 4 includes a monitor circuit portion 11 connected to the monitor electrode portion 8 of the transducer 3, an AGC circuit portion 12 connected to the monitor circuit portion 11, a driving circuit portion 13 connected to the AGC circuit portion 12, and an angular velocity detection circuit portion 14 connected to the sense electrode portion 10 of the transducer 3.
The monitor circuit portion 11 is composed of an amplifier into which is inputted the monitor signal 7 outputted from the monitor electrode portion 8, a band-pass filter (BPF) into which is inputted an output signal of the amplifier, a rectifier into which is inputted an output signal of the band-pass filter, and a smoothing circuit into which is inputted an output signal of the rectifier.
The AGC circuit portion 12 receives an output signal from the smoothing circuit in the monitor circuit portion 11, and according to the input signal, possesses the function of amplifying or attenuating an output signal from the band-pass filter in the monitor circuit portion 11. The driving circuit portion 13 outputs an amplified or attenuated output signal from the band-pass filter to the drive electrode portion 6 as the drive signal 5 used for driving the transducer 3. The angular velocity detection circuit portion 14 detects the angular velocity based on the sense signals 9 outputted from the sense electrode portion 10.
The transducer 3 is driven as it starts to oscillate upon input of the drive signal 5 from the drive electrode portion 6, and the oscillations are outputted from the monitor electrode portion 8 in the form of the monitor signal 7. The drive signal 5 is a sine wave with a specific frequency, and in a case where the amplitude of the drive signal 5 (a sine wave with a specific frequency) is different from the amplitude of the monitor signal 7 (a sine wave with the oscillation frequency induced by oscillations of the transducer 3), control is conducted in such a manner that the amplitude of the monitor signal 7 and the amplitude of the drive signal 5 coincide with each other.
More specifically, in a case where the amplitude of the monitor signal 7 is smaller than the amplitude of the drive signal 5, a correction signal to decrease the amplitude of the drive signal 5 is added to the drive signal 5, and in a case where the amplitude of the monitor signal 7 is larger than the amplitude of the drive signal 5, a correction signal to increase the amplitude of the drive signal 5 is added to the drive signal 5. These functions are performed by the AGC circuit portion 12 and the driving circuit portion 13, and oscillations of the transducer 3 are maintained at constant amplitude.
In the angular velocity sensor described above, the relationship among the drive signal 5, the monitor signal 7, and the sense signal 9 is shown in FIG. 12A, FIG. 12B and FIG. 12C. That is to say, with respect to the drive signal 5 which is a sine wave with a specific frequency as shown in FIG. 12A, the monitor signal 7 which is a sine wave with an oscillation frequency induced by oscillations of the transducer 3 is outputted in phase with the drive signal 5 as shown in FIG. 12B. As shown in FIG. 12C, the sense signal 9 which is a sine wave whose phase leads by 90 degrees is outputted in synchronization with the monitor signal 7 according to the angular velocity applied to the transducer 3.
As prior art document information related to the invention of the present application, for example, JP-A-2000-193459 is known.
With the above conventional configuration, a synchronous frequency synchronized with the monitor signal 7 and induced by the angular velocity applied to the transducer 3 is detected, and the synchronous frequency is outputted from the sense electrode portion 10 in the form of the sense signals 9. However, even when no angular velocity is occurring in the transducer 3, the sense signals 9 may be outputted because the occurrence of the angular velocity is detected erroneously depending on a mass balance of the transducer 3. This configuration therefore has a problem that the angular velocity cannot be calculated correctly. The mass balance of the transducer 3 referred to herein means, for example, the balance of masses of respective arm portions 2 for the tuning fork type transducer 3 in the shape of a letter U, a letter H or the like, and for a transducer other than the tuning fork type and shaped like a column or cone, it means the balance of mass in reference to the center of gravity.